The pyrethroids with which the present invention is concerned are crystallizable esters having at least one asymmetric carbon atom to which an epimerizable proton is attached. The more pesticidally active pyrethroids additionally contain at least one and usually two or more other asymmetric carbon atoms and therefore comprise isomeric mixtures wherein one or more of the isomers are more pesticidally active than the others. Representative of such pyrethroids are the alpha-cyanobenzyl esters of the formula (A): ##STR1## wherein R.sup.1 is halogen, haloalkyl, alkenyl or haloalkenyl; each R.sup.2 independently is halogen, alkyl, haloalkyl, alkoxy, phenyl, phenoxy, phenylalkyl, substituted phenyl and substituted phenylalkyl wherein the substituents include one or more of alkyl, halogen, haloalkyl, nitro, hydroxy and cyano; and n is 0-5, preferably 1-3. In the above formula the asymmetric carbon atoms are marked 1, 3 and alpha. All of the substituents on a host group may be the same, or the substituents may be different. Alkyl and alkoxy may contain 1-8 carbon atoms, preferably 1-4 carbon atoms. Alkenyl may comprise 2-8 carbon atoms, preferably 2-4 carbon atoms. Halogen includes fluorine, chlorine and bromine. A typical phenylalkyl group is benzyl. Substituted phenyl includes tolyl, xylyl, trichlorophenyl and trifluoromethylphenyl. Substituted phenylalkyl includes methylbenzyl, trichlorobenzyl and trifluoromethylbenzyl.
The foregoing and other pyrethroids are well known as disclosed, for example, in Kirk-Othmer, Encyclopedia of Chemical Technology, Second Edition, Vol. 13, pages 456-458, in the following U.S. Patents:
U.S. Pat. No. 4,024,163--Elliot et al (NRDC)
U.S. Pat. No. 4,133,826--Warnant et al (Roussel Uclaf)
U.S. Pat. No. 4,136,195--Warnant et al (Roussel Uclaf)
U.S. Pat. No. 4,213,916--Davies et al (Shell)
U.S. Pat. No. 4,287,208--Fuchs et al (Bayer)
U.S. Pat. No. 4,308,279--Smeltz (FMC)
U.S. Pat. No. 4,427,598--Mason et al (Shell)
U.S. Pat. No. 4,512,931--Robson (ICI)
U.S. Pat. No. 4,544,508--Fuchs et al (Bayer)
U.S. Pat. No. 4,544,510--Van Berkel et al (Shell)
U.S. Pat. No. 4,560,515--Stoutamire et al (Shell)
U.S. Pat. No. 4,582,646--Stoutamire et al (Shell)
U.S. Pat. No. 4,670,464--Doyle et al (ICI)
U.S. Pat. No. 4,681,969--Williams et al (ICI)
and in the following PCT patent publications:
WO 86/04215 - Hidasi et al (Chinoin)
WO 86/04216 - Hidasi et al (Chinoin)
All of the listed patents and publications are incorporated herein by reference.
Preferred pyrethroids convertible to more active isomers in accordance with the present invention are those of formula A wherein R.sup.1 is dihalovinyl or tetrahalopropenyl, R.sup.2 is phenoxy, and n is 1. The more preferred pyrethroids are those wherein n is 1, R.sup.1 is dihalovinyl or tetrahalopropenyl and R.sup.2 is phenoxy; and those wherein n is 2, R.sup.1 is dihalovinyl or tetrahalopropenyl and one R.sup.2 is fluorine and the other R.sup.2 is phenoxy. The latter preferred compounds are isomeric mixtures having the common name "cyfluthrin" when R.sup.1 is dichlorovinyl, n is 2 and one R.sup.2 is fluorine. When R.sup.1 is dichlorovinyl, n is 1 and R.sup.2 is phenoxy, the mixtures have the common name "cypermethrin."
Cypermethrin contains four cis and four trans isomers designated I-VIII as follows:
______________________________________ cis isomers I. (S)(.alpha.-cyano)(3-phenoxyphenyl)methyl 1R,cis-3- (2,2-dichloroethenyl)-2,2-dimethylcyclopropane- carboxylate (abbreviated 1R,cis S) II. (R)(.alpha.-cyano)(3-phenoxyphenyl)methyl 1S,cis-3- (2,2-dichloroethyl)-2,2-dimethylcyclopropane- carboxylate (abbreviated 1S,cis R) III. (S)(.alpha.-cyano)(3-phenoxyphenyl)methyl 1S,cis-3- (2,2-dichloroethenyl)-2,2-dimethylcyclopropane- carboxylate (abbreviated 1S,cis S) IV. (R)(.alpha.-cyano)(3-phenoxyphenyl)methyl 1R,cis-3- (2,2-dichloroethenyl)-2,2-dimethylcyclopropane- carboxylate (abbreviated 1R,cis R) trans isomers V. The trans isomer of I (abbreviated 1R,trans S) VI. The trans isomer of II (abbreviated 1S,trans R) VII. The trans isomer of III (abbreviated 1S,trans S) VIII. The trans isomer of IV (abbreviated 1R,trans R) ______________________________________
Cyfluthrin and other pyrethroids to which the invention is applicable comprise similar isomeric mixtures.
It is known that the most insecticidally active isomers of the foregoing eight isomers are I and V, and that enantiomer pairs I/II (abbreviated cis-2) and V/VI (abbreviated trans-2) are more insecticidally active than the enantiomer pairs III/IV (abbreviated cis-1) and VII/VIII (abbreviated trans-1 ). It is extremely difficult and commercially impractical to separate the more active isomers such as I and V from the complex isomer mixtures produced in the usual pyrethroid synthesis. Accordingly, efforts to produce more pesticidally active pyrethroids have focused on techniques for converting less active isomers in the synthesis product mixtures to more active isomers, i.e., to enrich isomeric mixtures with respect to the more active isomers, thus avoiding complex resolution procedures and the loss represented by discard of less active isomers.
Nevertheless, even when the isomeric mixtures have been converted rather than resolved, the conversion procedures have not been commercially practical because of poor yields, usually due to production of undesired by-product, often comprising as many isomers as the desired product, and because of time-consuming multiple steps, high temperatures and/or the need to recover expensive reagents. In the case of cypermethrin the major by-product is (R,S)-2-oxo-1,2-bis(3-phenoxyphenyl) ethyl cis- and trans-3-(2,2-dichloroethenyl)-2,2-dimethylcyclopropanecarboxylate, an eight isomer mixture commonly called the "benzoin by-product." Similar by-products are encountered in the synthesis of other pyrethroids such as cyfluthrin. Representative of prior efforts to convert isomer mixtures to more active species are the procedures disclosed in U.S. Pat. Nos. 4,213,916, 4,308,279, 4,544,510, 4,544,508, 4,512,931, 4,427,598, 4,670,646 and 4,681,969 and the two PCT patent publications cited above.